Astronaut Scott Kelly is preparing to return to Earth after spending an unprecedented year on the International Space Station. Kelly has been in the process of finishing his collection of samples, some of which will return with him in the Soyuz capsule when it lands near Kazakhstan. He’s also been packing up because, even though there is limited space in the capsule — it’s nearly 23 feet long and nearly 9 feet in diameter — astronauts can bring 1½ kilograms (about 3 pounds) of items back with them.

Most of Kelly’s items will be personal items that were given to him before he launched on March 27, 2015. The astronauts can also send other items back via SpaceX cargo spacecraft.

During his year on the space station, Kelly has been part of a twin health study comparing his condition, with the mental and physical toll of zero gravity, with that of his twin brother, retired astronaut Mark Kelly, who is on Earth. The data collection for the twin study goes on for three years, said Julie Robinson, NASA’s chief scientist for the ISS. The medical tests are measuring the impact of zero gravity on bone density, vision, the microbiome, blood, heart and cells, as well as the psychological impact on mood, stress and cognitive functions.

Kelly and his Russian counterpart, Mikhail Kornienko, are scheduled to land at Baikonur cosmodrome in Kazakhstan on Tuesday, March 1.